Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 441
PIC18F87J72 FAMILY
B.3.14 DITHERING
In order to suppress or attenuate the Idle tones present
in any Delta-Sigma ADCs, dithering can be applied to
the ADC. Dithering is the process of adding an error to
the ADC feedback loop in order to “decorrelate” the
outputs and “break” the Idle tones behavior. Usually a
random or pseudo-random generator adds an analog
or digital error to the feedback loop of the Delta-Sigma
ADC in order to ensure that no tonal behavior can
happen at its outputs. This error is filtered by the feed-
back loop and typically has a zero average value so
that the converter static transfer function is not dis-
turbed by the dithering process. However, the dithering
process slightly increases the noise floor (it adds noise
to the part) while reducing its tonal behavior, and thus,
improving SFDR and THD.
The dithering process scrambles the Idle tones into
baseband white noise and ensures that dynamic specs
(SNR, SINAD, THD, SFDR) are less signal dependent.
The AFE incorporates a proprietary dithering algorithm
on both ADCs in order to remove Idle tones and
improve THD, which is crucial for power metering
applications.
B.3.15 CROSSTALK
The crosstalk is defined as the perturbation caused by
one ADC channel on the other ADC channel. It is a
measurement of the isolation between the two ADCs
present in the chip.
This measurement is a two-step procedure:
1. Measure one ADC input with no perturbation on
the other ADC (ADC inputs shorted).
2. Measure the same ADC input with a
perturbation sine wave signal on the other ADC
at a certain predefined frequency.
The crosstalk is then the ratio between the output
power of the ADC when the perturbation is present and
when it is not divided by the power of the perturbation
signal.
A lower crosstalk value implies more independence
and isolation between the two channels.
The measurement of this signal is performed under the
following conditions:
•GAIN = 1
• PRESCALE = 1
• OSR = 256
• MCLK = 4 MHz
Step 1
• CH0+ = CH0- = SAVSS
• CH1+ = CH1- = SAVSS
Step 2
• CH0+ = CH0- = SAVSS
• CH1+ – CH1- = 1VP-P @ 50/60 Hz (full-scale
sine wave)
The crosstalk is then calculated with the following
formula:
EQUATION B-10:
B.3.16 PSRR
This is the ratio between a change in the power supply
voltage and the ADC output codes. It measures the
influence of the power supply voltage on the ADC
outputs.
The PSRR specification can be DC (the power supply
is taking multiple DC values) or AC (the power supply
is a sine wave at a certain frequency with a certain
common-mode). In AC, the amplitude of the sine wave
is representing the change in the power supply.
It is defined as:
EQUATION B-11:
Where VOUT is the equivalent input voltage that the
output code translates to with the ADC transfer
function. For the AFE, SAVDD ranges from 4.5V to
5.5V, and for AC PSRR, a 50/60 Hz sine wave is
chosen, centered around 5V, with a maximum 500 mV
amplitude. The PSRR specification is measured with
SAV
DD = SVDD.
CTalk dB 10
 CH0Power
 CH1Power
---------------------------------


log=
PSRR dB 20
 V
OUT
 SAVDD
----------------------


log=